Find the Focal Length of the Lens - PowerPoint Presentation

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Find the Focal Length of the Lens

A differentiated lab to test you skill with lab reports and geometric optics

Ray Tracing for Lenses

Lenses are used to focus light and form images. There are a variety of possible types; we will consider only the symmetric ones, the double concave and the double convex.

Ray Tracing for Lenses

If we think of a convex lens as consisting of prisms, we can see how light going through it converges at a focal point (assuming the lens is properly shaped).

Ray Tracing for Lenses

A concave lens can also be modeled by prisms:

Ray Tracing for Lenses

These diagrams show the principal rays for both types of lenses

:

The three principal rays for lenses are similar to those for mirrors:

The

P

ray—or parallel ray—approaches the lens parallel to its axis.

The

F

ray is drawn toward (concave) or through (convex) the focal point.

The midpoint ray (

M

ray) goes through the middle of the lens. Assuming the lens is thin enough, it will not be deflected. This is the thin-lens approximation.

Ray Tracing for Lenses

The convex lens forms different image types depending on where the object is located with respect to the focal point:

The Thin-Lens Equation

We derive the thin-lens equation in the same way we did the mirror equation, using these diagrams:

The Thin-Lens Equation

Sign conventions for thin lenses:

The focal Length can be found…

Using the equations seen yesterday, your single objective is to find the focal length of the convex lens that is in your group’s optics kit.

The way you communicate your findings will dictate your grade. There are several options to choose from.

The ‘D’ Option

This option will earn you up to a ‘D’ or 69%

You provide an answer for the focal length with the measurements and math. Minimal to no commentary on procedure and analysis.

f = 15.0

cm

The ‘C’ Option

This option will earn you up to a ‘C’ or 79%

You provide an answer for the focal length with the measurements and math. A clear and concise commentary is given on the procedure and analysis used.

The ‘B’ Option

This option will earn you up to a ‘B’ or 89%

You provide an answer for the focal length with the measurements and math. A clear and concise commentary is given on the procedure and analysis used. More than one pair of object and image distances are used to calculate the focal length.

Trial

#

d

o

d

i

f

1

2

3

The ‘A’ Option

This option will earn you up to an ‘A’ or 100%

You provide

an answer for the focal length with the measurements and math. A clear and concise commentary is given on the procedure and analysis used.

More than one pair of object and image distances are used to calculate the focal length. A graph of the data is shown to depict how the focal length can be visualized as the slope of a function relating d

i

and d

o

.

The Options for the Lab

Y

our single objective is to find the focal length of the convex lens that is in your group’s kit.

All of the levels allow you to do this, and they all build on each other. Ex: In order to do the B-level, you must be able to do the C-level workAdd a ray-diagram to any level for a polished look!

The

options for this lab

D –Level

Find an

answer for the focal length of your convex lens

C-level

Find a

focal length and explain how you obtained it.

B-Level

Use multiple data points to minimize

error in your focal point value.

A-Level

Use multiple data points and graphical analysis to minimize errors in

results

.